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National Advisory Committee for Aeronautics, Report - Scale Effect on Clark Y Airfoil Characteristics from NACA Full Scale Wind Tunnel Tests

Tests were conducted in the N.A.C.A. full-scale wind
tunnel to determine the aerodynamic characteristics of
the Clark Y airfoil over a large range of Reynolds Num-
bers. Three airfoils of aspect ratio 6 and with 4-, 6-,
and 8-foot chords were tested at velocities between 25 and
118 miles per hour, and the characteristics were obtained
for Reynolds Numbers (based on the airfoil chord) in
the range between 1,000,000 and 9,000,000 at the low
angles of attack, and between 1,000,000 and 6,000,000
at maximum lift. With increasing Reynolds Number the
airfoil characteristics are afiected in the following
manner: The drag at zero lift decreases, the maximum
lift increases, the slope of the lift curve increases, the angle
of zero lijt occurs at smaller negative angles, and the
pitching moment at zero lift does not change appreciably.
The Clark Y airfoil characteristics obtained from the
tests in the full-scale tunnel are compared with those from
the variable-density and the propeller-research tunnels,
and with the theoretical values. An analysis of the com-
parative experimental data indicates that the air stream
of the full—scale tunnel has a relatively low turbulence.
This inference is substantiated by the close agreement
obtained between the characteristics of airplanes measured
in the full-scale tunnel and those from flight tests, and by
sphere drag measurements that show the tunnel has a
turbulence similar to free air. It is therefore believed
that the efi'ects of turbulence on the characteristics of an
airfoil tested in the full-scale tunnel are small, and may be
neglected in applying the data to design.
The aerodynamic characteristics of airfoils ascer-
tained from different wind-tunnel investigations are
frequently not in agreement. The reasons for these
discrepancies are generally understood, having been
revealed partly by theory and partly through experi-
ment. The complete force equation, which includes
the terms expressing dynamic similitude, shows theo-
retically that comparable wind-tunnel results should
be obtained when airfoils having similar surfaces are
tested at the same Reynolds Number in wind tunnels
with like turbulences.